Tuning Thiophene with phosphorus: Synthesis and electronic properties of benzobisthiaphospholes

Article abstract of DOI:10.1002/chem.201402561

1,4-Dimercapto-2,5-diphosphinobenzene and 3,6-bis(hexyloxy)-1,4-dimercapto- 2,5-diphosphinobenzene were synthesized and combined with various acid chlorides to obtain a series of benzobisthiaphospholes. Electrochemical and photophysical properties of the substituted benzobisthiaphospholes have been evaluated, and the observed reductions are more facile than the related benzothiaphospholes and 2,6-diphenylbenzobisthiazole. A benzobisthiaphosphole with C₆H ₄-p-CN substituents was reduced at E_1/2=-1.08 V (vs. saturated calomel electrode (SCE)). X-ray diffraction data for several of these phosphorus heterocycles has been obtained, and DFT calculations at the B3LYP level have been performed. Fused-ring systems: Energetic tuning of conjugated building blocks can be achieved by atomic modulation of aromatic architectures. A phosphorus mimic of benzodithiophene and benzobisthiazole has been synthesized and structurally characterized (see figure). The UV/Vis spectrum of the phosphorus heterocycle is significantly redshifted compared to its organic counterparts. The redox properties of the benzobisthiaphospholes have been probed and revealed reversible reductions in THF.

Full text of DOI:10.1002/chem.201402561

DOI: 10.1002/chem.201402561
Full Paper
&
Phosphorus Heterocycles
Tuning Thiophene with Phosphorus: Synthesis and Electronic
Properties of Benzobisthiaphospholes
Yunyan Qiu,[a] Joshua C. Worch,[a] Danielle N. Chirdon,[a] Aman Kaur,[b] Andrew B. Maurer,[a]
Samuel Amsterdam,[a] Christopher R. Collins,[a] Tomislav Pintauer,[b] David Yaron,[a]
Stefan Bernhard,[a] and Kevin J. T. Noonan*[a]
Abstract: 1,4-Dimercapto-2,5-diphosphinobenzene and 3,6-
bis(hexyloxy)-1,4-dimercapto-2,5-diphosphinobenzene were
synthesized and combined with various acid chlorides to
obtain a series of benzobisthiaphospholes. Electrochemical
and photophysical properties of the substituted benzobis-
thiaphospholes have been evaluated, and the observed re-
ductions are more facile than the related benzothiaphosp-
holes and 2,6-diphenylbenzobisthiazole.
A benzobisthia-
phosphole with C6H4-p-CN substituents was reduced at
E
1/2 =À1.08 V (vs. saturated calomel electrode (SCE)). X-ray
diffraction data for several of these phosphorus heterocycles
has been obtained, and DFT calculations at the B3LYP level
have been performed.
often challenging synthetic targets.[3] The 1,3-benzoxaphosp-
hole and various derivatives are the only conjugated heterocy-
cles investigated to date with many of these compounds
being highly emissive in solution (Figure 1).[4] Studies that
expand upon the chemistry of phosphorus and explore its ver-
satility in conjugated frameworks are still in their infancy.
Herein, we describe the synthesis and electronic properties of
thiophene-based building blocks that incorporate P=C
bonds.[5]
Introduction
Phosphorus has emerged as a versatile element to modulate
the optoelectronic properties of conjugated organic materi-
als.[1] In particular, phosphole-based materials have attracted
a great deal of attention, because they often exhibit n-type be-
havior (Figure 1).[2] The phosphorus lone pair in phospholes
can also be chemically manipulated to modulate the HOMO–
LUMO gap and solid-state packing of conjugated architectur-
es.[2b] However, expansion to other phosphorus-based hetero-
cycles for organic electronics is still limited, because they are
The facile chemical modulation and remarkable charge-
transport properties of thiophene have made it the quintes-
sential heterocyclic ring for organic electronics.[6] The
incorporation of P=C bonds into a thiophene moiety
should lower the LUMO of the heterocycle and afford
n-type behavior for these systems (thiaphosphole,
Figure 1).[7] However, investigation of such species
has been limited to a few synthetic reports.[8] Herein,
we describe the first synthesis of the benzobisthia-
phosphole (BBTP, Figure 1). This compound is a struc-
tural mimic of benzodithiophene (BDT), a popular
building block for polymeric photovoltaic materials.[9]
BBTP is also isolobal with benzobisthiazole (BBT), and
BBT-based materials have been synthesized for tran-
sistors,[10] photovoltaics,[11] and nonlinear optics.[12]
The BBTP compounds were prepared with solubiliz-
ing hexyloxy groups and characterized by using X-ray crystal-
lography and cyclic voltammetry.
Figure 1. Conjugated building blocks for organic materials.
[a] Y. Qiu, J. C. Worch, D. N. Chirdon, A. B. Maurer, S. Amsterdam, C. R. Collins,
Prof. D. Yaron, Prof. S. Bernhard, Prof. K. J. T. Noonan
Department of Chemistry, Carnegie Mellon University
4400 Fifth Avenue, Pittsburgh, Pennsylvania, 15213 (USA)
E-mail: noonan@andrew.cmu.edu
[b] A. Kaur, Prof. T. Pintauer
Results and Discussion
Department of Chemistry and Biochemistry
Duquesne University, 600 Forbes Avenue, 308 Mellon Hall
Pittsburgh, Pennsylvania, 15282 (USA)
Synthesis of BTPs and BBTPs
Recently, we reported the preparation of benzothiaphospholes
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402561.
(BTPs 2a–e) from 1-mercapto-2-phosphinobenzene (1) and var-
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1,4-Dibromo-2,5-bis(hexyloxy)benzene (8) was synthesized
according to a literature procedure.[14] Compound 8 was com-
bined with tert-butyllithium, quenched with elemental sulfur,
and subjected to an acidic work-up to produce 9 (Scheme 3).
Deprotonation of 9 to form a dithiolate was achieved by using
LDA, and subsequent reaction with diisopropyl bromophos-
phate gave 10. Treatment of 10 with four equivalents of LDA
Scheme 1. Synthesis of BTPs from 1-mercapto-2-phosphinobenzene.
ious aryl acid chlorides (Scheme 1).[13] The reversible
one-electron reductions observed for BTPs highlight
the electron-accepting nature of the 1,3-thiaphosp-
hole moiety.[13a] The addition of a second thiaphosp-
hole moiety to the fused aromatic structure was ach-
ieved starting from 1,4-dibromobenzene (3).
1,4-Dibromobenzene (3) was treated with tert-bu-
tyllithium to induce lithium–halogen exchange. Ele-
mental sulfur was added to the dilithiobenzene and
the resultant dithiolate was quenched with bromodii-
sopropylphosphate to afford 4. Migration of the
phosphate group was achieved by combination of 4
with lithium diisopropylamide (LDA) at À788C fol-
lowed by slow warming to room temperature. An
acidic work-up of the reaction mixture gave 5, which
was subsequently reduced to 6 in 81% yield. The
combination of 6 with various aryl acid chlorides
gave either yellow or orange solids (Scheme 2). Un-
fortunately, compounds 7a–c are highly insoluble,
precluding structural characterization via solution-
phase H and 31P NMR spectroscopy. Mass-spectrome-
1
Scheme 3. Synthesis of hexyloxy-substituted BBTPs. LDA=lithium diisopropylamide;
ether=diethylether.
try data were obtained to confirm the formation of
7a–c, but neither electrochemical nor photophysical
characterization of these compounds was conducted.
Compounds 7d and e were characterized at 658C by using
1H and 31P NMR spectroscopy (see the Supporting Informa-
tion); however, the limited solubility still inhibited examination
of electrochemical and photophysical properties. To circum-
vent this issue, solubilizing hexyloxy groups were added to the
BBTP structure.
led to migration of the phosphate groups (11). Finally, reduc-
tion of 11 by using LiAlH4 in diethyl ether gave the desired 3,6-
bis(hexyloxy)-1,4-dimercapto-2,5-diphosphinobenzene
(12).
Several BBTPs were targeted by combining 12 with aryl acid
chlorides (Scheme 3). The reaction produced the desired com-
pounds 13a–e in moderate yields (53–79%) except for com-
pound 13c, which was obtained
in 22% yield. The compounds
have colors ranging from yellow
to orange and exhibit good solu-
bility in CH2Cl2, THF, and toluene.
The 31P NMR signals observed for
these
derivatives
(d=175–
195 ppm) are similar to the pre-
viously reported BTPs.[13a] Inter-
estingly, these signals are sig-
nificantly downfield compared
to similar annelated heter-
ophospholes, such as the 1,3-
benzazaphosphole (ꢀ70 ppm)
and the 1,3-benzoxaphosphole
(ꢀ85 ppm).[3a]
Scheme 2. Synthesis of BBTPs. Ether=diethylether.
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X-Ray crystallography
benzobisthiaphosphole in 13d is essentially co-planar with its
thiophene substituents (1.18). This has a direct effect on the
packing of these two structures. The measured distance be-
tween the planes of the central ring for adjacent molecules is
3.74 ꢁ in 13a, whereas in 13d, this distance is significantly
shortened to only 3.57 ꢁ. In fact, compound 13d exhibits face-
to-face slip stacking, which is illustrated in Figure 3.
Crystals suitable for X-ray diffraction analysis were obtained for
13a by slow evaporation of a toluene solution, whereas com-
pound 13d crystallized from a mixture of dichloromethane
and diethyl ether at À408C (Figure 2). The P=C bond lengths
Figure 2. Solid-state molecular structure of 13a and d. Thermal ellipsoids at
50% probability. Selected bond lengths [ꢁ] and angles [8] for 13a: S1ÀC4
1.721(5); S1ÀC2 1.745(5); P1ÀC3 1.754(4); P1ÀC4 1.713(5); C4-S1-C2 95.2(2);
C3-P1-C4 95.5(2); S1-C4-P1 117.9(3). Selected bond lengths [ꢁ] and angles [8]
for 13d: S1ÀC3 1.721(4); S1ÀC2 1.742(3); P1ÀC3 1.713(4); P1ÀC4 1.756(3);
C3-S1-C2 95.15(15); C3-P1-C4 94.71(15); S1-C3-P1 118.32(19).
Figure 3. Packing diagram of 13d along the c axis.
Photophysical properties
The absorption spectra of the BBTPs are consistent with an ex-
tended delocalized structure. Comparison with BTPs reveals
that the larger heterocycle induces a redshift of approximately
100 nm in the p–p* transition (447–471 nm for 13a–e com-
pared to 337–359 nm for 2a–e) and introduces a new absorp-
tion signal (Figure 4). TD-DFT calculations were used to help
assign the observed transitions (see the Supporting Informa-
tion).
for 13a (1.713(5); 1.754(4) ꢁ) and 13d (1.713(4); 1.756(3) ꢁ) are
comparable to the recently reported benzobisoxaphospholes
(1.694(1) and 1.782(1) ꢁ).[4e] The SÀC bond lengths for 13a
(1.721(5); 1.745(5) ꢁ) and 13d (1.721(4); 1.742(3) ꢁ) are quite
similar to the reported SÀC(sp2) bond lengths for benzobisthia-
zole 14 (1.736(2); 1.758(2) ꢁ) confirming the aromaticity of the
thiaphosphole (Scheme 4).[15]
Interestingly, the longest wavelength maximum of BBTP 13a
is significantly redshifted compared to its organic an-
alogues 14 and 15 (Scheme 4). Compound 13a has
three well-defined absorption bands at l=307, 358,
and 447 nm while 14 exhibits three absorption fea-
tures near l=330, 350, and 370 nm (in CHCl3).[16]
Compound 15 exhibits two absorption bands at 346
and 380 nm.[17] We suspect that the observed red-
shifting is primarily due to the insertion of the heavy
phosphorus atom as opposed to the hexyloxy substituents.
Though 7a is insoluble, the similar yellow color of 7a and 13a
suggests little redshifting from the alkoxy groups. Moreover,
DFT calculations predict only a 20 nm redshift in the p–p* tran-
sition from 7a to 13a. The BBTP derivatives are also signifi-
cantly redshifted compared to their phosphorus counterpart
16 (Scheme 4), which exhibits a p–p* transition at 337 nm.[4e]
Scheme 4.
Figure 2 illustrates the highly symmetric BBTP structure due
to the neighboring relationship of S and P in the periodic
table. The endocyclic bond angles about S and P in 13a (95.2
and 95.58) are nearly identical, whereas in 14, a large difference
about S and N was observed (88.9 and 110.88).[15] These differ-
ences illustrate the remarkable structural variation resulting
from exchange of isolobal fragments.
The structures in Figure 2 are best described with the central
benzobisthiaphosphole segment having either phenyl or 2-
thienyl substituents. Remarkably, the twist angles between the
fused core and the aromatic substituents differ substantially
for 13a and d. Compound 13a exhibits a 168 twist from copla-
narity for the central ring and its phenyl substituents while the
Electrochemistry
Electrochemical oxidation of the BTPs and BBTPs was observed
in CH2Cl2, whereas reduction was probed in THF (Table 1). All
of the compounds exhibited an irreversible oxidation. This pro-
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shifting and loss of intensity in consecutive potential cycles
(see the Supporting Information). To reproduce the oxidation
signal for those derivatives, the working electrode must be
polished in between cycles, which suggests that application of
highly positive potentials causes decomposition or other pro-
cesses creating deposits on the electrode. Oxidation can be
consecutively repeated at least four times without significant
change in all other derivatives. It is possible that electropoly-
merization of 13d and e is occurring with the formation of an
electrochemically inactive polymer. However, we currently
have no conclusive evidence for this process, and follow-up
studies are necessary to develop a better understanding of the
oxidative behavior of these compounds.
Upon scanning negative potentials, all of the benzothia-
phospholes undergo a quasi-reversible or irreversible first re-
duction, whereas 2c also exhibits a second reduction process.
Addition of hexyloxy groups and a second thiaphosphole
moiety resulted in facilitation of the first reduction process and
observation of a second reduction for all benzobisthiaphosp-
holes. Furthermore, the reversibility of the first reduction is
typically increased in 13a–e. Reductions cycle reproducibly for
all compounds when the anodic sweep does not exceed
+0.9 V.
Figure 4. UV/Vis absorption spectra of compounds 13a (black line) and 2a
(dotted line).
Table 1. Photophysical and electrochemical properties of 2a–e and
13a–e.
Abs.[a]
lmax [nm]/
e [cmÀ1 mÀ1
Em.[a] Ox.[b]
lem Epa
[nm]/ [V vs.
F [%] SCE]
Red. 1[e]
Epc
[V vs.
SCE]
Red. 2[e]
Epc
[mV] [V vs.
SCE]
DE
DE
[mV]
]
Reduction potentials of the compounds are not significantly
altered by using heterocyclic substituents (e.g., thiophene and
furan) instead of phenyl groups. However, as was previously
demonstrated for BTPs,[13a] reductions of phenyl substituted
BBTPs are readily tuned by modifying the phenyl ring. Com-
pound 13b displays the most negative reduction potentials in
its series due to the electron donating nature of its C6H4-p-
OMe substituents. The C6H4-p-CN group meanwhile has the
opposite effect causing 13c to exhibit the most positive reduc-
tion potentials, as well as a third reduction process (Figure 5).
The feasibility of using phosphorus to tune frontier orbitals in
heterocycles is also evident from the BBTPs. Comparison of the
electrochemical behavior of 13a with benzobisthiazole 14 re-
veals the expected effect of heavy heteroatom insertion. The
two reversible reductions observed for 14 in THF (À1.62 and
2a
2b
2c
2d
2e
337 (13500)
347 (20500)
344 (16400)
356 (15500)
359 (21000)
441
(4.3)
447
(5.4)
434
(1.0)
474
(4.2)
445
(2.9)
530
(0.3)
500
(1.5)
580
(0.3)
536
(0.8)
520
(0.8)
1.49
1.31
À1.73[f] 260
À1.84[f] 230
À1.36[f] 200
–
–
–
–
–
–
–
[c]
–
À1.88[g]
1.43
1.41
1.05
1.00
1.21
À1.71[g]
–
–
–
À1.76[f] 210
À1.46[h] 160
À1.59[h] 130
À1.08[h] 80
13a 447 (15300)
13b 447 (21100)
13c[i] 471 (8000)
13d 462 (16700)
13e 468 (24200)
À1.86[h] 180
À2.06[g]
–
À1.34[h] 80
1.05[d] À1.55[g]
–
À1.88[g]
–
1.05[d] À1.46[h] 150
À1.86[f] 220
[a] Absorbance and emission spectra were recorded in CH2Cl2 under N2,
and quantum yields were determined versus quinine sulfate. [b] Irreversi-
ble, measured under argon in CH2Cl2 at a scan rate of 100 mVsÀ1. [c] Pro-
cess is not clearly resolved from the solvent oxidation. [d] Process does
not cycle reproducibly due to interference with the working electrode.
Potentials are taken from initial scans. [e] Measured under argon in THF
at 100 mVsÀ1 with anodic sweeps not exceeding +0.90 V. [f] Quasi-rever-
sible. [g] Irreversible. [h] Reversible. [i] Compound 13c exhibited a third ir-
reversible reduction at À2.04 V.
cess is indicative of phospha-alkenes and occurs more readily
in the BBTPs than in their BTP analogues. The Epa values ob-
served for 13a–e range from 1.00–1.21 V (vs. SCE in CH2Cl2) so
oxidation is more facile than in benzobisoxaphosphole 16
(Epa =1.52 V vs. SCE in CH2Cl2).[4d] For BBTP derivatives 13d and
e, observation of the oxidation is complicated by significant
Figure 5. Cyclic voltammograms of 13a (black line) and 13c (dotted line)
measured in THF. Voltammograms were collected using 0.10m [N(nBu)4][PF6]
as the supporting electrolyte.
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À2.01 V vs. SCE)[18] are more negative than the related reduc-
tions of 13a (À1.46, À1.86 V vs. SCE) reflecting a decrease in
LUMO energy for the phosphole structure.
Acknowledgements
K.J.T.N. is grateful to the ARO for a Young Investigator Award
(63038-CH-YIP). The authors are also grateful to Prof. Roberto
Gil for help with NMR assignments. NMR Instrumentation at
Carnegie Mellon was partially supported by the National Sci-
ence Foundation (CHE-0130903 and CHE-1039870). S.B. and
D.Y. acknowledge support from the National Science Founda-
tion through CHE-1055547 and OCI-1135553. D.N.C. gratefully
acknowledges the support of the U.S. Department of Energy
Office of Science Graduate Research Fellowship.
To date, there are a very limited number of reports describ-
ing electrochemical studies of heterocyclic architectures bear-
ing more than one P=C bond. The benzobisoxaphosphole 16
has been characterized electrochemically and exhibits two re-
duction waves with the first one being reversible (E1/2
=
À2.01 V vs. SCE in THF). The exchange of O for S results in
a significantly more facile first reduction (E1/2 for 13a–e be-
tween À1.08 V and À1.59 V vs. SCE in THF). These data suggest
a significant decrease in the LUMO energy upon going from
the oxaphosphole to thiaphosphole moiety. Comparison of the
BBTP (13a–e) reduction potentials to the 2,2’-biphosphinine
(À1.85 and À2.42 V vs. SCE in DMF) also illustrates the ease of
reduction for the sulfur–phosphorus heterocycles.[7]
Keywords: fused-ring systems
acceptors · organic electronics · phosphorus heterocycles
·
heterocycles
·
n-type
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Conclusion
The synthesis and electrochemical properties of a series of
fused heterocycles (BBTPs: 13a–e) that incorporate P=C bonds
directly into the conjugation pathway have been reported. The
participation of the P=C bond in the delocalized p-electron
structure is indicated by substantial redshifts relative to their
BTP counterparts (2a–e) and to the analogous benzobisthia-
zole system (14). A potential advantage of these heterocycles
for materials applications is the more facile reduction. The re-
duction potentials of 13a are approximately 150 mV more
facile than their nitrogen counterpart 14. The results indicate
that insertion of phosphorus within the butadiene fragment of
thiophene can be used to alter both the electronic and physi-
cal structure of conjugated p-electron frameworks. Future
work will focus on incorporating the BBTP moiety into polymer
architectures.
Experimental Section
General procedure for the synthesis of BBTPs 13a, b, d,
and e
In a nitrogen-filled glovebox, a Schlenk flask (100 mL) was charged
with compound 12 (0.3 g, 0.74 mmol), dry toluene (30 mL), and an
aryl acid chloride (2.1 equiv, 1.55 mmol). The Schlenk flask was re-
moved from the glovebox, and triethylamine (4 equiv, 2.95 mmol)
was added. Then, the reaction vessel was immersed in an oil bath
at 1108C. The solution was stirred overnight and turned bright
yellow. After 17 h, the reaction mixture was cooled to RT, and
NaOH solution (20 mL, 1m) was added, and the mixture was stirred
for 2 min before the aqueous layer was removed by syringe. This
process was repeated once, and then MgSO4 was added to remove
residual water. The mixture was then transferred by cannula filtra-
tion into another 100 mL Schlenk flask. The solvent was removed
in vacuo and the resultant product was washed with degassed di-
ethyl ether (5 mL) under a N2 atmosphere.
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Published online on && &&, 0000
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FULL PAPER
Fused-ring systems: Energetic tuning
of conjugated building blocks can be
achieved by atomic modulation of aro-
matic architectures. A phosphorus
&
Phosphorus Heterocycles
Y. Qiu, J. C. Worch, D. N. Chirdon, A. Kaur,
A. B. Maurer, S. Amsterdam, C. R. Collins,
T. Pintauer, D. Yaron, S. Bernhard,
K. J. T. Noonan*
mimic of benzodithiophene and benzo-
bisthiazole has been synthesized and
structurally characterized (see figure).
The UV/Vis spectrum of the phosphorus
heterocycle is significantly redshifted
compared to its organic counterparts.
The redox properties of the benzobis-
thiaphospholes have been probed and
revealed reversible reductions in THF.
&& – &&
Tuning Thiophene with Phosphorus:
Synthesis and Electronic Properties of
Benzobisthiaphospholes
Chem. Eur. J. 2014, 20, 1 – 7
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ